Unwanted voltage surges have long been a critical problem to circuit designers of industrial and home electrical systems. Surges generated by load switching are often repetitive and range as high as 2,500 V. Lightning generated surges can range up to or over 6,000 V.
Surge protective devices have been made from SiC. It is also known that ZnO when mixed with certain additives and sintered into pellets, can exhibit non-linear V-I characteristics superior to SiC. These modified ZnO compositions are, therefore candidate materials for non-linear lightning arrester components and non-linear resistor applications. Such devices can have non-linearity due to contact between the individual grains of SiC or ZnO, i.e., completely due to electrical phenomenon within the bulk of the body.
The ZnO non-linear devices have been made by mixing additive oxides with ZnO powder, and then pressing and sintering, as taught by Matsuoka et al, in U.S. Pat. No. 3,663,458. In that patent, ZnO is mixed in a wet mill for 5 hours with additive materials such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, CoO and MnO to produce a homogeneous mixture. A binder such as water or polyvinyl alcohol can be added. The mixture was then molded at about 340 kg./sq. cm. (4,800 psi.) and single step sintered at 1,000.degree. to 1,450.degree. C for 1 to 3 hours, providing 1.3 cm. dia. by 0.05 to 0.25 cm. thick discs. Matsuoka et al, in U.S. Pat. No. 3,838,378, more thoroughly mixed ZnO in a wet mill for 24 hours with additive oxides and CeF.sub.3, to produce an extremely homogeneous mixture to which a binder could be added. The mixture was then molded at 250 kg./sq. cm. (3,500 psi) and single step sintered at 1,000.degree. to 1,450.degree. C for 1 to 10 hours, to provide bulk voltage non-linear bodies for lightning arresters, with dimensions as large as 3.5 to 4 cm. dia. and 2 cm. thickness.
Mixing the materials is one of the most important operations in making non-linear lightning arrester components and non-linear resistors, because the physical homogeneity of the product, and the reproducibility of the electrical characteristics, will depend on thorough mixing of the component powders. We have found that by merely milling or blend-mixing the ingredients, only a marginally acceptable product is produced, resulting in a large percentage of lightning arrester components and resistors being rejected due to varying electrical properties caused by lack of homogeneity. Also, most blend-mixed ZnO compositions require pressures of about 250 kg./sq. cm. to 2,140 kg./sq. cm. (3,500 psi. to 30,000 psi.) to consolidate the composition prior to sintering. These blend-mixed compositions appear to require high pressures and/or the use of expensive isostatic presses, to lessen the dramatic density gradient through the pressed body that otherwise exists after standard single or double action non-isostatic, uni-axial type pressing.
Organic binders, such as polyvinyl alcohol, have been used in the ceramic industry as an aid to granulation of ceramic oxide particles. Teter, in "Evaluation of Binders For Machinable Unfired Ceramics" RFP 659, Distributed by Clearing House For Federal Scientific And Technical Information, U.S. Dept. of Commerce Institute For Applied Tech., Dec. 10, 1965, described tensile strength testing of large ceramic bars made using Al.sub.2 O.sub.3, MgO or ZnO ceramic oxides with polyvinyl alcohol as a binder and using spray drying techniques. After wet milling the oxide-binder, the slurry was fed into a spray drying apparatus to provide granulated powder having an average particle size of 15 microns. The bars measured 1.9 cm. by 1.9 cm. by 15 cm., but Teter used isostatic pressing at 20,000 psi. to get a uniform compact. Waxes were then added to increase mechanical strength for machining.
It should be apparent that a method of producing ZnO sintered bodies, useful as surge protective devices such as voltage-non-linear resistors which are useful as lightning arrester components, and other type voltage limiters, using relatively low consolidation pressures, so that inexpensive graphite or steel dies can be used, and yet providing a uniform density gradient through the pressed body would be an advantageous advance in the art.
The method should be capable of producing larger type sintered bodies, having diameters of at least 2 in. (5 cm.) and a thickness of at least 1/4 in. (0.64 cm.), so as to be useful as stacked voltage non-linear components in lightning arresters. Preferably, bodies would be about 4 inches in diameter and 3 inches in thickness. Such thickness would allow the bodies to be cut to size for lightning arrester component applications, dramatically cutting down on the time-consuming pressing and sintering operations.